The background is described in connection with a integrated circuit charge pump of the type commonly employed in a wide array of electronic system designs and applications. It should be understood, however, that the principles disclosed may apply to designs where system board space is limited and precise current switching is a requirement.
In integrated circuit design, it is often necessary to employ a multi-level driver that provides voltages higher than the supplied voltage rails to the system. For example, it is common to float the voltage into the gate of an NMOS transistor at a level above the supply rail in order to keep the efficiency of the circuit up and make the drop across the high side NMOS small. A common technique for doing this is to build a charge pump on the chip or printed circuit board.
Standard integrated circuit charge pumps are readily available from a variety of manufacturers. In general, charge pump designs vary according to the size of the load and the amount of space available with a particular application. A charge pump may be either all on-chip or partially off-chip with the primary factor being the size of the charging capacitors. Where the size of the charging capacitors are large, a partially off-chip solution is often employed to accommodate the larger charging capacitors which require more current to reach the high voltage side of the charge pump.
Prior art charge pumps use one or more "blocking" diodes that allow charge to be transferred from a first capacitor, i.e. the pumping capacitor, to a second storage capacitor. A voltage level sufficient to drive a blocking diode is necessary to permit charging of the storage capacitor. This configuration is illustrated in FIG. 1, wherein a standard charge pump doubler circuit 10 is shown using blocking diodes D1 and D2.
In operation the blocking or pump diodes D1 and D2 are used to allow charge to be transferred from the pumping capacitor 14 (Cp) to a storage capacitor 20 (Cs). A driver circuit 11 comprising a buffer with a high current capability receives an oscillating input 30 that sets the switching rate of the diodes D1, D2. The diodes D1 and D2 are used to block current in one direction, while maintaining current flow into either the pump capacitor 14 (Cp) or the storage capacitor 20 (Cs).
Due to the drops across the diodes D1 and D2, however, the drop across the storage capacitor 20 can only approximately double the supply voltage Vs. Thus, a limitation inherent to the doubler configuration 10 is the losses due to forward drops on the blocking diodes in the charging path.
Additionally, the blocking diodes D1 and D2 have internal parasitics which steal current or charge from the capacitors 14 and 20 thus lowering the charge pump's efficiency. A schottky diode with a small forward drop may be used to limit such parasitics, but integrated schottkys are usually very large and area inefficient in high current applications. Additionally, the large series resistance of schottky diodes reduces the charge pump efficiency making their practicality limited to integrated applications.